We have applied an indirect immunofluorescence technique to cryostat sections of guinea pig cochleas using Met-enkephalin and Leu-enkephalin antisera. With both antisera, high immunofluorescence was seen in the inner spiral bundle, the tunnel spiral bundle and the intraganglionic spiral bundle. Immunoreactivity persisted to a dilution of 1/50 000. In three out of the twelve guinea pigs, immunoreactivity was observed under the outer hair cells, but only with the Met-enkephalin antiserum. Immunoreactivity of medium intensity was seen when the Met-enkephalin antiserum was pre-adsorbed by Leu-enkephalin. No immunoreactivity was seen when the Leu-enkephalin antiserum was pre-adsorbed by Met-enkephalin. After treatment of cochlear sections with acidic permanganate, immunoreactivity of medium intensity was still observed with the Leu-enkephalin antiserum. These observations indicate that Met-enkephalin and Leu-enkephalin co-exist in the cochlea in regions where efferent terminals are located, particularly in the inner spiral bundle below the inner hair cells.